1. Field of the Invention
The present invention relates to a compound control method for controlling a system, and particularly to a compound control method which is a combination of Proportional Plus Integral Plus Derivative Control (hereinafter referred to as "PID control") and Fuzzy Control.
2. Description of the Prior Art
In an automatic system control, the responsiveness and the stability are generally in a relationship contrary to each other. In other words, in order to obtain a speedy response in the control, the stability of the control has to be often sacrificed, and vice versa.
In a system where a PID control is most commonly used, if the gain is increased in order to obtain a speedy response, a hunting phenomenon or the like undesirable occurrence would cause the control system to be unstable.
On the other hand, the Fuzzy Systems Theory has been studied in various industrial fields recently. The studies on the Fuzzy System Theory and its applications have been actively pursued particularly in the fields of automatic system control.
Under the circumstance, there have been proposed several system control methods in which a combination of a PID control and a fuzzy control is used. A control method in which control parameters for the PID control are changed by a fuzzy inference has been proposed in, for example, Japanese Patent Application Laid-Open Gazettes 62-241,004, 62-241,005, 62-241,006, 63-123,102 and 63-247,801; and the Japanese magazine titled "KEISOH (Measurement and Instrument)", 1988, Vol. 31, No. 5, Pages 11 to 15 and 21 to 25.
In the methods disclosed in the above publications, input variables are first corrected by fuzzy controls and the corrected variables are used as PID control parameters. Then, only the outputs from the PID controls are used as the control variables to be inputted to the system to be controlled. The conventional methods, however, have a disadvantage in that the fuzzy control rules used in the fuzzy inference tend to become complicated and, therefore, some difficulties are liable to entail in their practical applications.